414
UNIT THREE
groups called
ganglia
(sing.,
ganglion
). On the other hand,
motor neuron cell bodies are typically in the gray matter of
the brain.
Numbers and names designate cranial nerves. The num-
bers indicate the order in which the nerves arise from the
brain, from anterior to posterior. The names describe pri-
mary functions or the general distribution of cranial nerve
F
bers
(f
g. 11.25)
.
The F
rst pair of cranial nerves, the
olfactory
(ol-fak
to-re)
nerves
(I), are associated with the sense of smell and include
only sensory neurons. These bipolar neurons, in the lining of
the upper nasal cavity, serve as
olfactory receptor cells.
Axons
from these receptors pass upward through the cribriform
plates of the ethmoid bone, carrying impulses to the olfac-
tory neurons in the
olfactory bulbs,
which are extensions of
the cerebral cortex just beneath the frontal lobes. Sensory
impulses move from the olfactory bulbs along
olfactory tracts
to cerebral centers where they are interpreted. The result of
this interpretation is the sensation of smell.
The second pair of cranial nerves, the
optic
(op
tik)
nerves
(II), are sensory and lead from the eyes to the brain
and are associated with vision. The cell bodies of these neu-
rons form ganglion cell layers in the eyes, and their axons pass
through the
optic foramina
of the orbits and continue into the
visual nerve pathways of the brain (see chapter 12, p. 475).
The third pair, the
oculomotor
(ok
u-lo-mo
tor)
nerves
(III), arise from the midbrain and pass into the orbits of the
eyes. One component of each nerve connects to a number of
voluntary muscles, including those that raise the eyelids and
four of the six muscles that move the eye.
A second portion of each oculomotor nerve is part of
the autonomic nervous system, supplying involuntary mus-
cles inside the eyes. These muscles help adjust the amount
of light that enters the eyes and help focus the lenses. This
nerve is considered motor, with some proprioceptive F
bers.
The term
general
in each of these categories indicates
that the F
bers are associated with general structures such as
the skin, skeletal muscles, glands, and viscera. Three other
groups of F
bers, found only in cranial nerves, are associated
with more specialized, or
special,
structures:
1.
Special somatic efferent F
bers
carry motor impulses
outward from the brain to the muscles used in chewing,
swallowing, speaking, and forming facial expressions.
2.
Special visceral afferent F
bers
carry sensory impulses
inward to the brain from the olfactory and taste
receptors.
3.
Special somatic afferent F
bers
carry sensory impulses
inward to the brain from the receptors of sight, hearing,
and equilibrium.
Cranial Nerves
Twelve pairs of
cranial nerves
arise from the underside of
the brain. Except for the F
rst two pairs, which begin in the
cerebrum, these nerves originate from the brainstem. They
pass from their sites of origin through foramina of the skull
and lead to areas of the head, neck, and trunk.
Most cranial nerves are mixed nerves, but some of those
associated with special senses, such as smell and vision,
have only sensory F
bers. Other cranial nerves that inner-
vate muscles and glands are primarily composed of motor
F bers and have only limited sensory functions. These are
neurons associated with certain receptors (
proprioceptors
)
that respond to the rate or degree of contraction of skeletal
muscles. These F
bers contribute directly to motor control, so
cranial nerves whose only sensory component is from such
proprioceptors are usually considered motor nerves. This
pertains to cranial nerves III, IV, VI, XI, and XII.
Neuron cell bodies to which the sensory F bers in the
cranial nerves attach are outside the brain and are usually in
Olfactory bulb
Hypoglossal (XII)
Vagus (X)
Vestibulocochlear (VIII)
Optic tract
Olfactory tract
Olfactory (I)
Optic (II)
Oculomotor (III)
Trochlear (IV)
Trigeminal (V)
Abducens (VI)
Facial (VII)
Glossopharyngeal (IX)
Accessory (XI)
FIGURE 11.25
The cranial nerves, except for the F
rst
two pairs, arise from the brainstem. They are identiF
ed
either by numbers indicating their order, their function, or
the general distribution of their F
bers.
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